Conventional television signals transmit video information in analog form by modulating the amplitude and frequency of a carrier signal. Digital television systems convert an analog video signal into digital information, which is transmitted by pulse modulating the amplitude and phase of a carrier signal. For example, in broadcasting high definition television (HDTV), digital information is transmitted by an 8 level amplitude modulation technique, known as 8VSB (vestigial sideband). In 8 VSB, modulation of a carrier to one of 8 levels (i.e., one of 8 symbols) defines 3 bits of digital information for each symbol clock interval. While analog television signals degrade gracefully in the presence of interference, digital broadcast systems can fail completely when the bit error rate overcomes the error tolerance of the system. Bit errors result from weak signals, noisy signals or signals subject to fading and distortion.
In an ideal radio wave propagation environment, there exists an unobstructed line-of-sight path between the transmitting and receiving antennas. Additionally, no other objects exist which may reflect the transmitted wave along another path to the receiving antenna. As is more often the case, however, there is no direct line of sight between the antennas. In an outdoor environment, natural or artificial obstructions, such as buildings, hills, and trees block the direct line of sight. Furthermore, these obstructions reflect the transmitted signal such that multiple versions with varying amplitudes, phases and time delays are simultaneously received.
The indoor environment is even more complicated since there is rarely an unobstructed path between the transmitter and receiver antennas. Furthermore, objects causing signal reflection and absorbtion are numerous and are often in motion.
Reception of the transmitted signal along multiple paths from the transmitter to the receiver causes signal distortion which manifests itself in a variety of ways. The different paths have different delays that cause replicas of the same signal to arrive at different times (like an echo) and sum at the receiver antenna, causing inter-symbol interference. The phases of these multipath signals may combine constructively or destructively resulting in large range of possible signal strengths. Additionally, this signal strength may vary with time or antenna location, and is known as signal fading. Signal fading can range from frequency selective fades to a flat fade over the entire frequency spectrum of interest. Indoor signals typically have severe multi-path distortions and are changing rapidly in time, ranging from flat fades to deep in-band nulls to relatively unimpaired signals. Conventional indoor TV antennas and outdoor antennas used with existing 8VSB receivers often do not produce reliable and uninterrupted reception of digital broadcast HDTV signals.
To mitigate the adverse effects of multiple path (multipath) distortion and signal fading, it is known to use a diversity receiver. In a diversity receiver, two (or more) independent antennas are used to receive two (or more) separate versions of the same signal. Each independent antenna provides a signal with different (ideally, uncorrelated) noise, fading and multi-path factors. These different signals may be obtained through various forms of diversity including spatial, temporal, polarization and direction-of-arrival diversity.
A diversity receiver has a plurality of receiver channels to process the plurality of antenna signals. By appropriate combining of the information extracted from each signal by each channel of the diversity receiver, a diversity receiver provides equal or superior performance compared to a non-diversity receiver operating on the “best” of the received signals alone.
In the prior art, the received signals in each of the individual receiver channels of a diversity receiver are processed separately and then combined. That is, each of the multiple receiver channels in a prior art diversity receiver is an independent receiver. Each of the respective diversity antenna signals is processed in one of the independent receiver channels of the diversity receiver.
Each receiver channel is independent in the sense that each includes a respective separate tuner, front-end function (such as for baud clock recovery and carrier recovery) and separate equalizer filter. The separate receiver channels of the prior art provide separate signal outputs, which are then combined in some manner.
In one prior art approach, the output of the separate diversity receiver channel having the stronger input signal (i.e., the higher signal to noise ratio) is selected over the output of the diversity receiver channel having the weaker input signal. In a second prior art approach, the outputs of the two diversity receiver channels are combined equally, regardless of input signal strength. In a third prior art approach, the outputs of the two receiver channels are combined in a maximal ratio combiner in accordance with the respective signal to noise ratio of each signal. In a maximal ratio combiner, the receiver channel with the highest signal to noise ratio provides the greatest contribution to the final output. In general, a prior art diversity receiver processes the received diversity signals in separate receiver channels with regard to receiver functions such as tuning, automatic gain control (AGC), baud clock recovery, RF carrier recovery, and forward equalization.